The liquid reduction-oxidation or redox process is used to scrub hydrogen sulphide (H.sub.2 S) containing gas streams. In this process the gas containing H.sub.2 S is contacted with an alkaline liquid phase (usually at pH 7.0 to 11.0) containing a dissolved metal organic chelate reagent. While any polyvalent transition metal can be employed in this process, iron is most commonly used. The H.sub.2 S from the gas stream is absorbed into the alkaline solution forming sulphide ions: EQU H.sub.2 S.sub.(g) .fwdarw.H.sub.2 S.sub.(aq) .fwdarw.H.sup.+.sub.(aq) +HS.sup.-.sub.(aq) .fwdarw.2H.sup.+.sub.(aq) +S.sup.2-.sub.(aq)( 1)
The sulphide ions react with the polyvalent metal, oxidizing the sulphide to elemental sulphur and reducing the oxidation state of the metal: EQU 2(Fe.sup.3+.chelate).sub.(aq) +S.sup.2-.sub.(aq) =2(Fe.sup.2+.chelate).sub.(aq) +S.sup.0.sub.(s) ( 1)
The metal is then oxidized with dissolved oxygen in the same or a separate vessel: EQU 2(Fe.sup.2+. chelate).sub.(aq)+H.sub.2 O+O.sub.2(aq) =2(Fe.sup.3+. chelate).sub.(aq) +2OH.sup.-.sub.(aq) ( 3)
The overall sulphur producing reaction is: EQU H.sub.2 S.sub.(g) +O.sub.2(g) .fwdarw.H.sub.2 O.sub.(l) +S.sup.0.sub.(s)( 4)
Side reactions oxidize some dissolved sulphide ions to acidic sulphur compounds, primarily sulphate (SO.sub.4.sup.2-) and thiosulphate (S.sub.2 O.sub.3.sup.2-). Sulphite (SO.sub.3.sup.2-) and dithionate (S.sub.2 O.sub.6.sup.2-) are also formed, but are normally present in insignificant quantities. Walter, C. M., indicated in U.S. Pat. No. 4,009,251 that above pH 7.0 from 2 to 9% of the H.sub.2 S fed to a liquid redox scrubber was converted into acidic sulphur compounds. These sulphur compounds tend to lower the pH of the scrubber solution reducing its scrubbing effectiveness. It is normal practice in these processes to maintain the pH of the scrubber solution above 7.0 by the continuous or periodic addition of alkaline chemicals such as the ammonium or alkali metal salts of carbonate (CO.sub.3.sup.2-), bicarbonate (HCO.sub.3.sup.-), or hydroxide (OH.sup.-).
The neutralization of the acidic sulphur compounds results in a steady build up of the corresponding ammonium or alkali metal salts. The build up of salts is described by the overall chemical equations given below (for the two most common species formed): EQU 2H.sub.2 S.sub.(aq) +2O.sub.2(aq) +2MOH.sub.(aq) .fwdarw.M.sub.2 S.sub.2 O.sub.3(aq) +3H.sub.2 O (5) EQU M.sub.2 S.sub.2 O.sub.3(aq) +2MOH.sub.(aq) +2O.sub.2(aq) .fwdarw.2M.sub.2 SO.sub.4(aq) +H.sub.2 O (6)
The presence of excess dissolved salt in the scrubber solution can contaminate the elemental sulphur produced. In scrubbers where the sulphur is removed by settling, high concentrations of dissolved salts can lead to poor sulphur settling because of high solution specific gravity and/or viscosity. If no action is taken the concentration of these salts will increase until the solution becomes saturated. The salts will then precipitate out of solution potentially plugging the piping of the unit.
Several different approaches have been taken to solve the problem of salt build up in the liquid redox processes. Thompson, R. B. in U.S. Pat. No. 4,218,342 recommended withdrawing and disposing a portion of the scrubber solution, and replacing this "blowdown" with fresh, salt-free solution to undersaturate the system. When a blowdown is taken it eliminates all components of the solution, including those which are beneficial or necessary for the operation of the scrubber. Various organic chemicals (such as EDTA, NTA, IDA, sorbitol etc.) are added to the solution to complex the polyvalent metal reagent and increase its solubility at the pH of the system. Additional chemicals are added to the scrubber solution to promote sulphur settling (flocculants, surfactants, antifoam agents), to destroy bacteria (biocides), and to control pH (KOH, KHCO.sub.3, K.sub.2 CO.sub.3, NaOH, NaHCO.sub.3, Na.sub.2 CO.sub.3, etc.). In removing and replacing scrubber solution, all these beneficial chemicals are sacrificed to bleed the offending salts.
The preferred approach would be to withdraw only salts and recycle all of the active chemicals in the solution back to the scrubber. In U.S. Pat. No. 4,859,437 Grinstead, R. R., suggests that a polymeric chelating agent with molecular weight between 500 and 1,000,000 be used to complex the reactive metal (e.g. iron). Grinstead claims that it is then possible to remove water, and all dissolved species having a molecular weight less than 500 using ultrafiltration or diffusion dialysis. Any beneficial chemicals dissolved in the scrubber solution with molecular weight less than 500 (such as those which control pH) will be removed with the salts and degraded organics. Obviously this process restricts the practitioner to using only polymeric chelating agents with suitably high molecular weights.
It is the object of the present invention to provide a process utilizing an electrodialysis system which will:
i) selectively remove alkali metal salts of thiosulphate (S.sub.2 O.sub.3.sup.2-) and sulphate (SO.sub.4.sup.2-) from hydrogen sulphide (H.sub.2 S) scrubber solution of the liquid redox type; PA1 ii) retain in the scrubber solution for subsequent recycle:
a) the polyvalent metal reagent (e.g., iron), PA2 b) the organic chelating agent, without placing any restriction on the nature (i.e. molecular weight) of the chelating agent to be used in the H.sub.2 S scrubber, PA2 c) and pH adjusting chemicals.
The use of electrodialysis to remove inorganic ions from solutions containing chelated polyvalent transition metals has been reported. Walker, R. J., reported in U.S. Pat. No. 4,820,391 the use of electrodialysis to regenerate SO.sub.2 -NO.sub.x scrubber solution containing Fe.sup.2+. EDTA (ferrous-ethylene diamine tetra acetic acid complex) by the removal of sulphite (SO.sub.3.sup.2-), bisulphite (HSO.sub.3.sup.-) and dithionate (S.sub.2 O.sub.6.sup.2-) ions. Ono and Watanabe described a process to remove silver (Ag.sup.+) and silver.thiosulphate complex ions from a solution containing both Fe.sup.2+.EDTA and Fe.sup.3+.EDTA in U.S. Pat. No. 4,256,559. In European Patent No. 0015737 A1, Grenda, D. W., revealed an electromembrane process to regenerate electroless copper plating baths by the removal of formate (HCO.sub.2.sup.-) and sulphate (SO.sub.4.sup.2-) ions through an anion selective membrane, from a solution containing Cu.sup.2+ chelated by a variety of different organic chemicals. However, the use of electrodialysis to remove alkali metal salts of sulphate (SO.sub.4.sup.2-), and thiosulphate (S.sub.2 O.sub.3.sup.2-) from hydrogen sulphide (H.sub.2 S) scrubber solutions of the liquid redox type has never been reported.
Natural gas containing significant quantities of H.sub.2 S ("sour gas") must be treated to prevent corrosion of pipelines during transport. This type of gas is often treated in a two step process. In the first step, H.sub.2 S and other acid gases such as SO.sub.2, CO.sub.2 and mercaptans are removed from the natural gas in an absorber containing an amine solution. This solution is thermally regenerated forming a concentrated gaseous product referred to as "acid gas". This acid gas is then treated in a Claus-type process for the removal of H.sub.2 S and SO.sub.2, a liquid redox scrubber for the removal of H.sub.2 S, or is incinerated. The amine solution used to separate the acid gas from the natural gas, can become contaminated with a variety of salts. In European Patent No. 0 286 143 A1 Gregory, R., reported the use of electrodialysis for the removal of inorganic salt species from alkanolamine absorber solution.